Pelicans tuck their wings and free-fall straight down when diving for fish. Suppose a pelican starts its dive from a height of 15.0 m and cannot change its path once committed.

If it takes a fish 0.25 s to perform evasive action, at what minimum height must it spot the pelican to escape? Assume the fish is at the surface of the water.

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  1. The fish must spot the pelican when it is 0.25 s (or more) from hitting the water.

    The pelican takes
    t = sqrt(2H/g) = 1.75 s to hit the water if it starts its dive at H = 15 m.

    Its vertical drop 1.50 s after starting the dive is y = (1/2)g*(1.50)^2 = 11.03 m
    It is 15 - 11.03 = 4.97 m above the water surface at that time,

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